Updated Schema and samples to use @data-type instead of @class

The Schema for HTMLBook; the htmlbook.html and Alice in Wonderland samples in the samples/ directory; and the specfication.asciidoc document have been updated to reflect the switchover to use the custom "data-type" attribute for semantic inflection (i.e., \<section data-type="chapter"\>) instead of "class", which is now free to be used for whatever custom semantics users would like.

The htmlbook-xsl change still needs to be updated accordingly as well, so I'll be doing that next.